1. Field of the Invention
The present invention generally relates to input devices, and more particularly to an input device using a capacitive sensor-based input unit for coordinate data input.
2. Description of the Related Art
In general, a notebook computer is provided with an input pad as a coordinate data input device. One known coordinate data sensor of the input pad is a capacitive sensor.
The capacitive sensor-based input pad includes X-electrodes formed at a predetermined interval and Y-electrodes formed at a predetermined interval, which face each other across an insulation layer having a predetermined permittivity. Once a finger or a conductor such as a pen touches a protective sheet which overlays either of the X- or Y-electrodes, the capacitances between the X-electrodes and the Y-electrodes vary at the location where it touches the protective sheet.
The input pad includes a controller for calculating the peak in the variation of capacitances between the X- and Y-electrodes to generate X-Y coordinate data from the resulting value, which is then fed to a main controller of a computer.
In the capacitive sensor-based input pad, however, the variation of capacitances between the electrodes exhibits different sharpness depending upon the area of the portion of protective sheet which the conductor touches. For example, when a small area of the protective sheet is touched with a pen or a fingertip, the variation of capacitances depicts a sharply curved profile. On the other hand, when a larger area of the protective sheet is touched such as with a finger pad, the variation of capacitances depicts a gently curved profile. When a palm unintentionally touches the protective sheet during a keyboard operation, the capacitances also vary with a modest varying rate. As a result, problems occur as follows.
First, once a palm touches an input pad as mentioned above, coordinate data is output from the input pad although it is not intended to use the input pad for coordinate data input. Second, while a finger pad touches a larger area of the protective sheet and rests thereon, the peak in variation of capacitances changes over time, resulting in a sequential output of finely changing coordinate data. Therefore, a cursor or the like appears to finely oscillate on a computer screen.
Accordingly, it is an object of the present invention to provide an input device which prevents coordinate data from being output, for example, when a palm unintentionally touches an input pad.
It is another object of the present invention to provide an input device capable of generating coordinate data so as to provide a less noticeable oscillation of a cursor on a screen when the variation of capacitances depicts a gently curved profile.
To this end, in an aspect of the present invention, an input device includes an input unit that detects the variation of capacitances at a plurality of detection points Ni (i is an integer), and a control unit that calculates detection outputs from the plurality of detection points Ni to determine the peak position P in the variations of capacitances to generate coordinate data from the peak position P. The calculation is performed using the maximum value detected at any one of the detection points Ni, the previous value detected at at least one previous point, and the following value detected at at least one point which follows the detection point at which the maximum value is obtained. When the maximum value transitions relative to the previous and following values by less than a first threshold value, the control unit does not output the coordinate data.
Therefore, for example, when a palm unintentionally touches the input device, the coordinate data is not output, and an erroneous detection can be avoided.
In another aspect of the present invention, an input device includes an input unit that detects the variation of capacitances at a plurality of detection points Ni (i is an integer), and a control unit that calculates detection outputs from the plurality of detection points Ni to determine the peak position P in the variation of capacitances to generate coordinate data at a fixed time interval from the peak position P. The control unit performs a correction between the peak position P determined at a given point and coordinate data M obtained at the previous point such that a corrected value which is smaller than (P-M) is added to the coordinate data M. Further, the calculation is performed using the maximum value detected at any one of the detection points Ni, the previous value detected at at least one previous point, and the following value detected at at least one point which follows the detection point at which the maximum value is generated to make a correction. The corrected value varies such that when the maximum value changes by less than a second threshold value relative to the previous and following values the corrected value is less than the corrected value when the maximum value changes by more than the second threshold value.
The input device according to the present invention allows for a coordinate detection with a high accuracy when the variation of capacitances exhibits a gently curved profile.
Preferably, the correction in which the corrected value is made smaller is performed when the displacement of the peak position P which is determined at a fixed time interval is within a predetermined distance.
For example, when a finger rests on the input unit, the oscillation of a cursor on a screen is suppressed.
The corrected value may be calculated by subtracting (Mnxe2x88x921) from Pn or a function of Pn and by dividing the resulting value by a predetermined integer m, where Pn indicates the peak position determined at a given time, and (Mnxe2x88x921) indicates the coordinate data determined at the previous time.
The correction may be performed using the integer m as a predetermined value when the displacement of the peak value P which is determined at a fixed time interval is within a predetermined distance. The correction using the same integer m or a different integer may be additionally performed when the maximum value transitions relative to the previous and following values by less than the second threshold value.
Preferably, the value calculated from (2Bxe2x88x92Axe2x88x92C) is compared to any one of the threshold values, where B denotes the maximum value at any of the detection points Ni, A denotes the previous value, and C denotes the following value.
The coordinate of the peak position may be defined by the following equation:   P  =            (              coordinate        ⁢                  xe2x80x83                ⁢        of        ⁢                  xe2x80x83                ⁢        B            )        -                  A        -        C                    2        ⁢                  (                                    2              ⁢              B                        -            A            -            C                    )                    